Tectonic fibrous veins: initiation and evolution. Ouachita Orogen, Arkansas

Veins are ubiquitous features in deformed rocks. Despite observations on syntectonic veins spanning two centuries, fundamental questions remain unanswered. Their origin as fractures is largely established but it is still not known why these fractures initiate where they do and how the vein evolves once started. We studied veins from the Lower Ordovician Mazarn Formation in the Arkansas? Ouachitas combining textural observations, stable isotopes, fluid inclusions, SEM-based cathodoluminescence and electron back-scattered diffraction to understand the initial stage of vein formation, its later evolution, the role of fluids and their environment of formation. The veins are located at boudin necks and are synchronous with cleavage formation. Texturally, veins are characterized by veinlets (thin veins between 5 and 25 ?m thick) that parallel the vein-host interface and fibers (columns of quartz or calcite) perpendicular to the vein-host interface between 30 and 350 ?m wide. Veinlets are localized fractures filled with quartz. The crystallographic orientation of the precipitated material in veinlets is inherited from host grains at the micron scale and replicated as fibers? lengths grow to centimeters. The vein-forming fluid was cyclically supersaturated yet never very far from saturation. ?18O values of vein quartz and host are within 2? of each other suggesting that the fluid was rock-buffered. Nevertheless, ?18O and ?13C define a ?J? shaped trend. Although it is not possible to date any portion of this curve, the simplest explanation is that the fluid evolved from rock-buffered in a closed system to fluid-dominated in an open system. The range of pressure-temperature conditions of vein formation is between 275 and 385 ?C and 1100 and 3400 bars, from fluid inclusions and quartz-calcite oxygen isotopes thermometry. By examining a vein from tip to middle, we have established a sequence of events from inception to maturity in vein growth. Vein formation starts with folding followed by flattening of resistant sandstone layers which in turn gives rise to boudinage. Boudinage formation allowed for fracture localization along boudin-necks. The vein grew by the repeated addition of veinlets in the neck region. Recrystallization later modified the fibers by obliterating some evidence of the veinlets and moving fiber walls.